The present invention relates to reservoirs, and more particularly, to fluid reservoirs for use in gas turbine engines.
In many gas turbine engines, a fluid reservoir is used to store lubricating oil for engine components. A supply pump attached to a supply passage takes fluid from the fluid reservoir to the engine components, and a scavenge pump attached to a scavenge passage returns the fluid from the engine components to the fluid reservoir.
Typically, the scavenge pump returns a quantity of air along with the fluid. When the engine is shut down, the supply pump and scavenge pump also slow down and eventually stop. During the slow down, lubricated, moving components of the engine also slow down, and the fluid on those components tends to fall to scavenge pick-up locations more quickly. This effect allows the scavenge pump to return more fluid to the fluid reservoir than the supply pump removes from the fluid reservoir during the slow down. Additionally, when a lubrication system is pressurized, the system typically depressurizes during the shut down. This causes compressed air in the scavenge passage to expand, forcing additional fluid to enter the fluid reservoir during and immediately after shut down.
After shut down, any fluid remaining in the scavenge passage eventually settles at the lowest point and the remaining air rises. The scavenge passage tends to hold a relatively large quantity of air leading into the fluid reservoir when it is shut down. When the engine is started up again, the supply pump pumps fluid out of the fluid reservoir. Because the scavenge passage is filled mostly with air at that time, the scavenge pump, essentially, pumps air into the fluid reservoir to replace the space previously occupied by fluid. When this happens, the fluid level in the fluid reservoir drops, sometimes substantially. This drop in the fluid level is sometimes called “gulp.” Fluid reservoirs are typically designed to be large enough to hold enough fluid to accommodate the “gulp.” However, a larger fluid reservoir undesirably adds weight to the entire system.